<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>操纵css</title>
  <style>
    #div1{
      width: 200px;
      height: 200px;
      background-color: #f00;
    }
</style>
</head>
<body>
<div id="div1"></div>

<script>
  let div1 = document.getElementById("div1");
  div1.onclick = function(){
    div1.style.backgroundColor = '#0f0';
    div1.style.width = '300px';
  }
</script>
</body>
</html>